Two component adhesive systems have been described in the patent literature, as represented particularly by U.S. Pat. No. 3,812,003 and No. 3,935,051, as well as being available commercially for a decade or more from the adhesive suppliers. In general these adhesives comprise a prepolymer component and a curative component which are generally mixed in ratios of about 4:1 or higher to give essentially a highly fluid mix unless the mix is highly loaded with filler. Typically the prepolymer component is the reaction product of an organic polyisocyanate with a reactive hydrogen containing material containing mostly two hydroxyls and having a molecular weight of 500 to about 5000 or higher, preferably selected from the polyester polyols and the polyether polyols. The curative component preferably is a low molecular weight polyfunctional polyol preferably having three or more hydroxyls of relatively low equivalent weight. Also these adhesives generally contain sufficient urethane catalyst to give the desired gel time to permit the adhesive to be applied at least to the substrate before the adhesive gels. The prepolymer usually contains about 2 to about 10 percent and preferably 4 to about 7 percent by weight of free NCO.
Since these adhesives without filler inherently have low sag resistance those of ordinary skill in this art have incorporated fillers to develop or increase the sag resistance where said resistance is needed. Unfortunately this technique also increases the viscosity of the component or components containing the filler and makes it more difficult to mix the components and then spread the adhesive on a substrate. Hence loading with high levels of filler needed to achieve sag resistance results in difficulties in getting satisfactory mixing because high pressure mixing or high shear equipment and high pressure pumping equipment is needed. Further, high loading in many cases tends to lower the strength of the adhesive bond and therefore is undesirable from that standpoint.
U.S. Pat. Nos. 3,714,127 and 3,886,122 teach to add primary diamines to reduce moisture sensitivity of the polyurethane adhesive, but do not teach the use of the secondary diamines to get sag resistance.